d3d12: add DXGI winsys
This merge-request does two things:
- Implements support for D3D12 specific DXGI-swapchain based winsys. This allows us to perform zero-copy rendering to a swapchain instead of doing a CPU copy.
- Adds a libgl-d3d12 target that always uses the D3D12 driver and doesn't implement the OpenGL32.DLL interfaces. This allows us to reduce the code-size of the driver quite a bit.